Engine timing systems typically include at least one driving sprocket located on the engine crankshaft and at least one driven sprocket on an engine camshaft. The rotation of the crankshaft causes the rotation of the camshaft through an endless power chain transmission.
More complicated engine timing systems connect a crankshaft with two or more shafts by a pair of chains. The crankshaft includes two sprockets. Each chain is connected to one or more driven sprockets, including sprockets on each of the two overhead camshafts. Typically, the chain systems in more complicated engine timing systems will include tensioners on the slack side of each chain to maintain chain tension and snubbers on the tight side of each chain to control chain movement during operation.
Some engine timing systems have two (or dual) overhead camshafts for each bank of cylinders. The dual camshafts on a single bank can both be rotated by connection to the same chain. Alternatively, the second camshaft can be rotated by an additional camshaft-to-camshaft chain drive. The cam-to-cam drive chain can also include single or dual tensioners for chain control.
Some engine systems, such as three cylinder engines, due to the number of cylinders and arrangement of the cylinders are inherently unbalanced. In these engines, balance shafts are employed to balance the inherent inbalance of the engine. Since the balance shafts are driven by the crankshaft, torsional vibrations and oscillations along the crankshaft may be transferred to the balance shafts and through the chain drive and create unnecessary high chain tensions throughout the engine timing system and accessory drive.
The rotating crankshaft may undergo resonance at certain frequencies. Since the balance shafts are coupled to the crankshaft by one or more balance shaft chains, the balance shafts are directly exposed to these extreme resonant torsional oscillations. Vibrations from the resonance of the crankshaft are often transferred throughout the system, including the balance shafts and can significantly increase the load on the systems and components, increase the noise from the engine, and increase wear and fatigue loading of the timing chains and components.
Conventional approaches to this problem have focused on reducing rotational perturbations of the crankshaft by means of internal devices such as Lanchaster dampers and harmonic balancers. External devices such as fluid engine mounts and engine mounts having adjustable damping characteristics have also been used. By contrast, the present invention focuses on absorbing the torsional vibrations of a crankshaft by using a torsionally compliant sprocket system on the crankshaft to absorb the crankshaft torsional vibrations and prevent their transfer to other parts of the engine system.
Some prior art timing systems use a rubber damper piece placed against a sprocket and bolted to the shaft to absorb vibrations. However, the rubber damper piece may fracture from the extreme resonance vibrations. Other timing systems employ a weight that is positioned on the shaft and held against the sprocket by a Belleville washer. Frictional material is also placed at the area of contact between the sprocket and the weight to absorb vibrations. These systems, while effective at damping have drawbacks in terms of production, assembly and durability.
An example of the above-described prior vibration damping techniques is found in Wojcikowski, U.S. Pat. No. 4,317,388, which issued on Mar. 2, 1982. That patent discloses a gear with split damping rings of diameter slightly smaller than the gear bolted to each side of the gear with a tapered bolt and nut assembly. Tightening of the bolt cams the damping ring outward, producing pressure circumferentially against the rim of the gear and causing tensile stress on the gear. Additionally, tightening of the bolts presses the elastomeric washers associated with the bolt and nut assembly firmly against the web of the gear which damps the stress wave passing from the rim through the web and into the shaft. In contrast to this prior art structure, the present invention utilizes a novel arrangement of sprockets to produce a torsionally compliant sprocket assembly to reduce the transfer of vibrations of the crankshaft to other parts of the engine system.
Another example of the above-described prior art damping techniques is Funashashi, U.S. Pat. No. 5,308,289, which issued on May 3, 1994. The damper pulley disclosed therein consists of a pulley joined to a damper mass member with a resilient rubber member. The pulley and the damper-mass member each have at least two projections, and the projections of the pulley contact the sides of the projections of the damper mass member. A second resilient rubber member is placed between the contacting projections. Bending vibrations from the crankshaft cause the pulley to vibrate in the radial direction and the first resilient rubber member deforms, causing the dynamic damper to resonate with the pulley and restrain the bending vibrations. Torsional vibrations cause the pulley to vibrate in the circumferential direction. The second resilient rubber member undergoes compression deformation, decreasing the spring force and raising the resonance frequency against the torsional vibrations. The present invention avoids the use of rubber which has wear problems in use.
Another example of a prior damping technique is Kirschner, U.S. Pat. No. 4,254,985, which issued on Mar. 10, 1981. That patent discloses a damping ring for rotating wheels that includes a viscoelastic damping material disposed within an annular groove in the surface of the wheel. A metal ring is positioned in the groove at the top of the damping material. In operation, the damping material undergoes shear deformation.